High efficiency, peak-power reducing, domestic hot water heater

ABSTRACT

There is herein described a peak-power reducing domestic hot water heater comprising: a closed tank having a predetermined water holding capacity, a hot water outlet in a top end wall of said tank, a cold water inlet in a side wall of said tank adjacent a bottom wall thereof, three spaced apart resistive heating elements projecting substantially horizontally in said tank, each of said resistive heating elements having a temperature sensing element to enable a control of their respective activation; a bottom one of said resistive heating elements extending in said tank spaced slightly above said bottom wall and having a lowest power rating, said bottom resistive heating element being turned on when water is flowing into said cold water inlet and drawn out of said hot water outlet during a peak-power demand time period; a middle one of said resistive heating elements extending in said tank at a level which is calculated to be equal or above an average maximum daily water consumption volume drawn out during the peak power demand time periods; and a top one of said resistive heating elements extending between said middle element and said top end wall of said tank and having a highest power rating; said middle and said bottom resistive heating elements being activated simultaneously only when said top resistive heating element is inactive.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a Continuation-In-Part of U.S. patent application Ser. No. 11/556,798, entitled “High efficiency, peak-power reducing, domestic hot water heater” filed on Nov. 6, 2006.

TECHNICAL FIELD

The present invention relates to a high efficiency, peak-power reducing, domestic hot water heater provided with three resistive heating elements.

BACKGROUND ART

In U.S. Pat. No. 4,948,948, there is described a water heater with multiple heating elements having different power factors and wherein these elements are controlled by a control circuit so that the elements are switched on at different periods of a day, outside peak hours, in order to reduce the power loads on an electrical distribution network during peak electrical power demand periods.

During peak hours when hot water is used, normally between 6:00 a.m. and 9:00 a.m. and 6:00 p.m. to 9:00 p.m., there is an excessive demand of power on the electrical distribution network. Electrical utilities have been searching for adequate solutions to this problem and one such solution is to increase the cost of electricity during peak periods of time thereby forcing consumers to use hot water at different periods of time whereby to try and reduce the demand during peak power periods. Another solution is for utilities to control the domestic circuits branched to high power rated appliances during this peak period of time and such controls have to be done remotely or with timers. These solutions are, however, costly to the utilities and are not popular with consumers. They also cause very high instantaneous demand at re-activation if too many units are turned back on at the same time. The U.S. Pat. No. 4,948,948 referred to hereinabove also discusses other attempts by utilities to control power consumption during peak demand periods. These other attempts are however limited and are the cause of various other problems.

As described in U.S. Pat. No. 4,948,948 the resistive elements are of different power ratings, with the top one of the elements being the highest power rated for heating a small volume of water in the top portion of the reservoir where water is drawn out of the tank to maintain the water in top portion at the set hot water temperature. However, during periods of peak demand, the amount of water in the top portion of the reservoir is quickly exhausted as it has been found that many consumers will draw hot water during a single peak period of the day rather than two separate periods, which therefore requires a much larger hot water volume.

SUMMARY

It is therefore a feature of the present application to provide a high efficiency, peak-power reducing, domestic hot water heater which addresses issues associated with the prior art.

Accordingly, there is provided a high efficiency, peak-power reducing, domestic hot water heater having three spaced-apart resistive heating elements which is an improvement of the hot water heater described in the afore-mentioned U.S. patent.

According to an embodiment, there is further provided a high efficiency, peak-power reducing, domestic hot water heater having three spaced-apart resistive heating elements mounted in a tank having a capacity of 270 liters and 20 inch or less diameter and wherein a low watt density resistive heating element is mounted at the bottom of the tank.

According to another embodiment, there is further provided a high efficiency, peak-power reducing, domestic hot water heater and wherein the middle resistive heating element is disposed at a level exceeding an average maximum water consumption volume of water drawn during a peak-power demand time period.

According to yet another embodiment, a high efficiency, peak-power reducing, domestic hot water heater is described. It comprises a closed tank having a predetermined water holding capacity. A hot water outlet is provided in a top end wall of the tank. A cold water inlet is provided in a side wall of the tank adjacent a bottom wall thereof. Three spaced apart resistive heating elements project substantially horizontally in the tank. A bottom one of the resistive heating elements extends in the tank spaced slightly above the bottom wall. A middle one of the resistive heating elements extends in the tank at a level calculated at approximately an average maximum water consumption volume of water drawn during a peak power demand time period. A top one of the resistive heating elements extends between the middle element and the top end wall of the tank. The bottom element has a low watt density in the range of from about 15 to 30 W/in².

According to still another embodiment, there is provided a method of reducing the kilowatt demand of a domestic hot water heater during peak hour periods without reducing the amount of hot water requested by a user using a large volume of hot water between 90 to 220 liters from a hot water tank having a predetermined water holding capacity of 180 or 270 liters. The method comprises the steps of providing a hot water tank with three spaced-apart electrical resistive heating elements extending therein in a spaced-apart manner. The bottom one of the resistive heating elements has a low watt density rating in the range of from about 15 to 30 W/in². A middle one of the resistive heating elements is positioned at a level calculated at approximately an average maximum water consumption of between about 90 to 130 liters dependent on the tank size of about 180 or 270 liters.

According to yet another embodiment, the above method further comprises operating the middle resistive heating element to maintain a water temperature at its level to approximately 140° F. to form a barrier in the hot water tank to reduce the propagation of harmful bacteria from the bottom of the tank towards a top portion of the tank where hot water is drawn.

In accordance with still another embodiment of the present application, there is provided a peak-power reducing domestic hot water heater comprising: a closed tank having a predetermined water holding capacity, a hot water outlet in a top end wall of the tank, a cold water inlet in a side wall of the tank adjacent a bottom wall thereof, three spaced apart resistive heating elements projecting substantially horizontally in the tank, each of the resistive heating elements having a temperature sensing element to enable a control of their respective activation; a bottom one of the resistive heating elements extending in the tank spaced slightly above the bottom wall and having a lowest power rating, the bottom resistive heating element being turned on when water is flowing into the cold water inlet and drawn out of the hot water outlet during one of the daily peak-power demand time periods; a middle one of the resistive heating elements extending in the tank at a level which is calculated to be equal or above an average maximum daily water consumption volume drawn out during the peak power demand time periods; and a top one of the resistive heating elements extending between the middle element and the top end wall of the tank and having a highest power rating; the middle and the bottom resistive heating elements being activated simultaneously only when the top resistive heating element is inactive.

Yet another embodiment provides a method of reducing the kilowatt demand of a domestic hot water heater during peak hour periods without reducing the amount of hot water requested by a user from a hot water tank having a predetermined water holding capacity, the method comprising the steps of: providing a hot water tank with three spaced-apart electrical resistive heating elements extending therein in a spaced-apart manner and each having a thermostat to enable a control of their respective activation; positioning a middle one of the resistive heating elements at a level exceeding an average maximum daily water consumption volume drawn during one of the peak hour periods, and activating the bottom one of the resistive heating elements when water is flowing into the cold water inlet and drawn out of the hot water outlet during the peak hour periods, and simultaneously activating the middle and the bottom one of the resistive heating elements only when the top resistive heating element is inactive.

Further details of these and other embodiment will be apparent from the detailed description and accompanying figures.

BRIEF DESCRIPTION OF DRAWINGS

Reference is now made to the Figures, in which:

FIG. 1 is a graph from the Florida Solar Energy Center which shows a comparison between different fractional daily hot water draw profiles as compiled by various organizations.

FIG. 2 is a schematic sectional view showing a hot water tank constructed in accordance with the prior art; and

FIG. 3 is a comparable schematic sectional view of a high efficiency, peak-power reducing, domestic hot water tank in accordance with an embodiment;

FIG. 4 is another schematic sectional view of a high efficiency, peak-power reducing, domestic hot water tank in accordance with another embodiment;

FIG. 5 is a graph showing typical diversified electrical power demand for typical resistance water heaters and heat pump water heaters;

FIG. 6 is a block diagram showing a switching apparatus used in a testing of the tank of FIG. 3 or 4, and a testing of a typical design of a two-element hot water heating tank;

FIG. 7 is a graph showing a bin destitution of daily hot water consumption for the tank of FIG. 3 or 4, where about 5,000 data points are measured and averaged over all the customers involved in the testing;

FIG. 8 is a graph of an average cumulated daily hot water usage per customer involved in the testing;

FIG. 9 is a graph illustrating all of the collected hot daily water usage profiles for all the customers involved in the testing of the typical two-elements design;

FIG. 10 is a graph illustrating all of the collected hot daily water usage profiles for all the customers involved in the testing of the tank of FIG. 3 or 4;

FIG. 11 is a graph of the diversified power demand for the tank of FIG. 3 or 4 compared to the diversified power demand for a two-element resistive water heater tank;

FIG. 12 a is a graph showing the mean, maximum and minimum during the daily peak power demand period between 7 h-11 h in the morning, for to a two-element resistive water heater tank;

FIG. 12 b is a graph showing the mean, maximum and minimum during the daily peak power demand period between 7 h-11 h in the morning, for the tank of FIG. 3 or 4;

FIG. 13 a is a graph showing the mean, maximum and minimum during the daily peak power demand period between 19 h-21 h in the evening, for to a two-element resistive water heater tank; and

FIG. 13 b is a graph showing the mean, maximum and minimum during the daily peak power demand period between 19 h-21 h in the evening, for the tank of FIG. 3 or 4.

DESCRIPTION OF PREFERRED EMBODIMENTS

Although it is stated in the above referred-to U.S. patent that any size of tank can be used, one must be careful in the actual position of the elements in order to insure that end users do not run out of hot water and that the intermediate element does not activate unless absolutely necessary. Furthermore, it is not recommendable that the actual size of the tank be less than 270 liters in cold climate geographical areas due to the fact that a smaller tank will not provide the appropriate amount of hot water requested by a user during peak periods. Warmer climates where tank inlet water temperatures are higher than in northern areas of North-America, could most likely allow smaller volume tanks such as 180 liters. Nonetheless, hot water drawing trends must be looked at. As such, the following table exposes figures of the average hot water consumption. These were taken from ASHRAE Standard.

Average Hot Water Use, L Hourly Daily Weekly Monthly Group OVL Peak OVL Peak OVL Peak OVL Peak All families 9.8 17.3 236 254 1652 1873 7178 7700 “Typical” 9.9 21.9 239 252 1673 1981 7270 7866 families

One can easily see that the peak water consumption must be taken into account as to eliminate the possibility of lacking hot water. Thus, the amount for typical daily hot water consumption can be set at a minimum amount of 254 liters per day.

Another study realized by the Florida Solar Energy Center clearly indicates that the consumption is slightly more. The graph of FIG. 1 compares different fractional daily hot water draw profiles compiled by various organizations.

Knowing that the graph of FIG. 1 indicates fractional draws, one must take into consideration the size of the tank in use. Assuming that the water heater in use is a 270 liter tank, by extrapolating the values in the graph to the volume of the tank, we obtain the following table.

ASHRAE Becker Fraction of Volume Fraction of Volume daily hot according to daily hot according to Hour water draw tank size water draw tank size  1 0.009 2.430 0.006 1.620  2 0.009 2.430 0.003 0.810  3 0.009 2.430 0.001 0.270  4 0.009 2.430 0.001 0.270  5 0.009 2.430 0.003 0.810  6 0.010 2.700 0.022 5.940  7 0.075 20.250 0.075 20.250  8 0.075 20.250 0.080 21.600  9 0.065 17.550 0.077 20.790 10 0.065 17.550 0.067 18.090 11 0.065 17.550 0.061 16.470 12 0.047 12.690 0.049 13.230 13 0.047 12.690 0.042 11.340 14 0.038 10.260 0.038 10.125 15 0.038 10.260 0.034 9.180 16 0.038 10.260 0.038 10.260 17 0.038 10.260 0.044 11.880 18 0.063 17.010 0.058 15.660 19 0.063 17.010 0.069 18.630 20 0.063 17.010 0.065 17.550 21 0.063 17.010 0.059 15.930 22 0.051 13.770 0.049 13.230 23 0.051 13.770 0.043 11.510 24 0.009 2.430 0.024 6.480 Total 1.009 272.430 1.008 272.025

From this table, we can clearly see that an entire tank usually used up in one day by an average family. However, care must be taken in interpreting this as this profile may not cover exceptions. One must keep in mind that it is crucial to avoid lack of hot water particularly where people tend to concentrate their consumption in one peak period of a day rather than two, i.e. morning and nighttime. As such the hot water drawn can be as high as 130 to 150 liters. Therefore, in order to always satisfy demand and to avoid increasing peak power demand from the utilities the volume of water to be heated by the bottom element should be set between 130 and 150 liters, for instance at 130 liters.

Other hot water consumption studies have been done. For example, a study by the Canadian Building Data and Analysis Centre titled “Domestic Water Heating and Water Heating Energy Consumption in Canada” (C. Aguilar, D. J. White and David L. Ryan, Domestic Water Heating and Water Heater Energy Consumption in Canada, CBEEDAC 2005-RP-02, April 2005) found relevant studies on household energy demand for Hot Water. <<According to Wiehagen and Sikora (2002a), a 1985 study that monitored hot water use for 59 residences in Canada found average hot water use per household to be 236 liters per-capita consumption values ranging from 47 to 86 liters per day . . . >>.

Per Household measure Source Location Liters/day Wiehagen and Sikora Canada 236 (2002a) [Perlman and Mills - Henze (2002) US 227.4 DeOreo (2000) US 246.8 Since there does not appear to be any information on energy consumption for water heating purposes by Canadian households, it is necessary to utilize U.S. values. . . . Household Values Underlying US DOE (Department of Energy) calculations for year 2000 in the following Table >>.

Average Average Hot Water Heater Household Water Use Fuel Type Size (Liters/day) Electricity 2.45 171.5 Natural Gas 2.82 188.9 LPG 2.58 173.0 Oil 2.68 178.1 U.S. Average 178.1

The present water heater is therefore developed to raise the utilization factor of a load and reduce the peak demand of the water heater, which is designed as a three elements water heater.

Referring now to FIGS. 2 and 3, there is shown an example of a 20 inch diameter 270 liters water heater tank with FIG. 2 representing the tank of the prior patent referred to.

FIG. 3 represents the high efficiency, peak-power reducing, domestic hot water tank in accordance with an embodiment.

Although different sizes of tanks can be used, the tank is usually chosen to be as high as possible in order to take full advantage of the stratification effect of hot and cold water in the tank in order to reduce water temperature diffusion. In other words, a shorter, stubbier tank of equal volume may have more difficulty to deliver as much hot water as a taller, slimmer one. Furthermore, the water heater is adapted to be capable of meeting the required specifications of the CSA delivery test stated in C191 Standards.

Although the previous design is made to meet the demand of a typical consumer, variations of this can be made for people who consume more or less water. The actual positions of the elements are all related proportionally to the volumes of water to be heated as per the extrapolations and statistics demonstrated herein. For example, one could design a smaller tank for people that consume very little hot water, such as about 175 liters, (38.5 US gallons). When extrapolating the values from the graph of FIG. 1, we obtain the following table.

ASHRAE Becker Fraction of Volume Fraction of Volume daily hot according to daily hot according to Hour water draw tank size water draw tank size  1 0.009 1.575 0.006 1.050  2 0.009 1.575 0.003 0.525  3 0.009 1.575 0.001 0.175  4 0.009 1.575 0.001 0.175  5 0.009 1.575 0.003 0.525  6 0.010 1.750 0.022 3.850  7 0.075 13.125 0.075 13.125  8 0.075 13.125 0.080 14.000  9 0.065 11.375 0.077 13.475 10 0.065 11.375 0.067 11.725 11 0.065 11.375 0.061 10.675 12 0.047 8.225 0.049 8.575 13 0.047 8.225 0.042 7.350 14 0.038 6.650 0.038 6.563 15 0.038 6.650 0.034 5.950 16 0.038 6.650 0.038 6.650 17 0.038 6.650 0.044 7.700 18 0.063 11.025 0.058 10.150 19 0.063 11.025 0.069 12.075 20 0.063 11.025 0.065 11.375 21 0.063 11.025 0.059 10.325 22 0.051 8.925 0.049 8.575 23 0.051 8.925 0.043 7.525 24 0.009 1.575 0.024 4.200 Total 1.009 176.575 1.008 176.313

We can see once again that peak period consumption is relatively high. Using the same reasoning as before, we can assume that a below average user that concentrates his use of hot water during one period rather than two could use about 90 liters. Thus, the intermediate element would have to be positioned as to never be activated before approximately 90 or more liters of water are consumed.

From the above study one can therefore conclude that a consumer who draws hot water during a single peak demand period will draw from anywhere between 90 to 130 liters or more.

Thus, with reference to FIG. 2, it can be seen that with the 270 liter prior art tank 10, the central resistive heating element 11 would be rendered operative as it is disposed at a level of about 130 liters. Accordingly, as soon as the consumption approaches 130 liters the central element is switched on therefore increasing power demand on the power distribution network.

Also, the top resistive element 12 is located at a level in the uppermost portion of the tank and heats approximately 12 gallons of water, but that element has a large power rating of 3800 watts to 4500 watts or more. However, this element is only turned on when the heater is plugged in for the first time, or if the consumer uses up most of the hot water in the tank.

The lowermost resistive element 13 has a power rating of 800 watts or less, but that element is on most of the time and because of this, the lifespan of that element is much shorter than the other elements and therefore must be designed accordingly to increase its lifespan.

As previously mentioned, the purpose of the three resistive elements at their respective locations inside the tank and their control circuit, is to heat the water in priority during very low power demand periods.

For example, between 8:00 p.m. and 7:00 a.m., the water is slowly heated to a usable hot water temperature. Therefore, during morning and early evening peak power demand periods, there should be ample hot water to be drawn from the tank. However, if, as described hereinabove, many consumers only demand hot water during only one peak period of time during a day, thereby demanding much larger volumes at a time, the supply of hot water from a hot water heater may not be adequate and the top resistive heating element is switched on, thereby increasing the load on the network of the utility. Thermostat accuracy must also be considered because if they are not precise from one to the other, the resistive heating elements will not be activated at the right time, thus not meeting delivery tests.

FIG. 3 shows the improved design of the domestic hot water heater of the present invention. A tank 20 similar to the one as described in the prior art is used, namely a tank of 270 liters, having a diameter of about 20 inches. In this case however, the middle resistive heating element 21 is disposed as near as possible or slightly above the average maximum water consumption volume drawn during a single peak hour demand time period. This maximum water volume is identified by the level 22. Level 22 may vary depending on a cumulated daily water consumption volume taken only during both of the peak power demand time periods which is also referred to as an averaged maximum daily water consumption volume taken during the peak power demand time periods. Accordingly, the middle resistive heating element 21 will not be activated by a consumer drawing a volume of hot water during a single peak demand time period which is lower or corresponding to the averaged maximum daily water consumption volume described above.

Another feature of the high efficiency, peak-power reducing, domestic hot water heater 20 of the present invention is that the bottom heating element 24 optionally has a low watt density rating in the range of from about 15 to 30 W/in². It is also disposed close to the bottom wall 25 of the tank, similar to that as shown in FIG. 2.

It is to be noted here that a cold water inlet 26 is connected to the side wall 27 and adjacent the bottom wall 25 whereby not to create too much turbulence at the bottom of the tank, as is the case with top entry tanks where the feed tube extends in the tank vertically downwards from the top wall 28 of the tank. Turbulence causes water to mix in large volume taking more time to supply hot water, thus reducing the amount of hot water available, at the top of the tank and entraining bacteria in the top region of the tank where hot water is drawn.

In both tanks 10 and 20, the hot water outlet 29 is connected to the top wall to draw the hot water in the top portion 30 of the tank. Also, the top resistive heating element 25 extends substantially at the same height as with the prior art tank 10 and, as herein shown, is at a level wherein there are approximately 46.21 liters of water thereabove for a 270 liter tank or within 30-50 liters.

It is further pointed out that the positions of the resistive heating elements 21, 23 and 24 also provide for efficient water temperature stratification within the tank thereby reducing diffusion of the cold water introduced at the bottom of the tank in a non-turbulent manner. This ensures that the top portion 30 of the tank has an adequate supply of hot water.

Further, the middle heating element produces a heat barrier in the water within the tank, and the temperature in the vicinity of the middle resistive heating element is in the range of about 140° F. This reduces the propagation of harmful bacteria such as legionnela to the hot water top portion 30 of the tank.

The tank as shown in FIG. 3 is a 270 liter tank and the bottom heating element extends at a water level of about 20 liters from the bottom. The middle resistive heating element 21 extends at a water level of about 130 liters from the bottom wall 25 whereas the top heating element extends at a water level of about 220 liters from the bottom wall 25. As previously described this top resistive heating element 23 is at a distance of about 46 or less liters from the top wall 28.

The bottom resistive heating element is of a low watt density rating to have an extended lifespan, as this element will operate almost all of the time to maintain the set hot water temperature of the tank.

This bottom resistive heating element 24 has a metal sheath which can be made of copper, incoloy or stainless steel or other suitable materials. The resistance is typically made of a nickel-chromium alloy coil surrounded by magnesium oxide powder or other suitable materials. It is also important that the surface watts load of the coil be kept at a minimum to increase its life. Although many means can be used to accomplish this, one means is to increase the gauge of the resistance wire.

It is noted that the resistive heating elements may also have a polymer core in accordance with other suitable heating element design configurations.

Typical domestic water heaters are equipped with screw-type elements that have a watt output value ranging anywhere from 1500 to 5500 watts. The surface load density, expressed in watts per square inch of tube surface, usually ranges from 80 to 130 W/in². This surface load is also present on the resistance inside of the element and ranges usually from 2 to 4 times the surface load of the tube. It is known that the higher the surface load of the coil as well as the tube, the shorter is the expected life of the element.

Furthermore, these elements are designed to operate in cycles. They heat the water on demand for the period of time required for it to heat the water to the desired temperature. The higher the output rating of the element (Watts), the shorter it will take for it to heat a definite volume of water. Inversely, the lower the output, the longer it will take to heat the water.

Since it is desirable that the bottom element 24 lasts just as long as the tank itself, it must be built for that purpose. The most important factor that must be taken into consideration is that the lower resistive heating element 24, which is rated at 800 watt, will operate continuously for a very long period of time until the water in the tank reaches the set thermostat temperature. As such the following criteria are respected: the bottom resistive element 24 has an increased life and therefore has a very low watt surface load of 15-30 W/in²), a heavy resistance gauge (1-2 wire gauge oversize), is of premium quality material for the resistance (High grade Nickel-Chromium Alloy), is of resistor style design, bolt-on instead of screw-in attachment, and has a premium quality grade MgO (magnesium oxide powder), and fusion welds on pin-coil assemblies and electrical contacts.

One can therefore appreciate that the high efficiency, peak-power reducing, domestic hot water heater tank 20 as described herein has features which produce beneficial results and one of these features is the position of the middle resistive heating element which creates a second tempered zone in a predetermined area of the tank which lies around or above the average maximum water consumption volume drawn during the two peak power demand time periods, on a daily basis per typical household usage. That middle element also provides a temperature barrier of about 140° F. thereby preventing bacteria from migrating to the top hot water portion of the tank. Further, the bottom resistive heating element is rated to outlast the life of the tank as it operates most of the time to maintain the set hot water temperature of the tank.

The above described hot water tank can have, for example and as illustrated in FIG. 4, three resistive heating elements (23, 21 and 24) each controlled through their dedicated thermostat (32, 33 and 34 respectively), and spaced apart so as to accommodate for various peak demand hot water consumption profiles.

The bottom resistive element has a lowest power rating set at about 800 Watts or lower. This bottom heating element is activated as soon as there is an admission of cold water into the cold water inlet 26 and that hot water is drawn out of the tank through the hot water outlet 29. Such an activation permits a heating of the water inside the tank using a low power demand over a long period of time. Considering a 270 L tank and an average household hot water daily demand determined to be about 194 to 220 L, the bottom heating element functions almost continuously.

The middle heating element has a power rating set at about 3000 Watts or lower and is located at and or above the average maximum water consumption volume drawn during the peak power demand time periods, which can be calculated from the average maximum household daily consumption of hot water as detailed hereinabove. The top heating element has the highest power rating and the highest priority compared to the other two heating elements. The power rating of the top element is set at about 3800 Watts or lower.

For a 270 L tank capacity and an average daily hot water consumption around 200 L per day, the lowest element is positioned such that the 800 Watts is activated during the utility peak demand time periods, thereby reducing the power demand below the typical diversified power demand of 1 to 1.4 kW.

In addition, the bottom resistive element and the middle resistive element are activated simultaneously only when the top resistive heating element is inactive or not heating.

The following describes tests which were performed on the above described embodiment of the high-efficiency, peak-power reducing, domestic hot water tank.

On a daily basis, diversified residential water heater loads generally have two peaks, one in the morning and one in the afternoon, as illustrated in the graph of FIG. 5. Although the installed resistive elements of standard water heaters typically reaches between 3.8 kW to about 5.5 kW, the diversified demand in the peaks tends to coincide approximately with those of electric utilities and typically varies from 1.0 to 1.4 kW. FIG. 5 shows the diversified demand for typical resistive water heater from Northeast Utilities in a year 2002 study. The highest curve being for the Resistance Water Heater type (resistive) and the lowest one with heat pump water heater (taken from NYLE Heat-pump water heater evaluation—Final report—January 2002, Submitted to Northeast Utilities, AIL Research, Inc.). By taking advantage of the thermal storage of water heaters, the control strategies are to smooth out the demand curves to eliminate short-term extremes. This reduces the average cost of electricity by improving the load factor, reducing the need for generation capacity by shifting electricity use from peak demand to off-peak demand time periods. Since it is now becoming more difficult to justify the construction of new power generation stations as well as transport and distribution lines, load management is being considered by electric utilities.

The tests were conducted on hot water tanks especially designed for the purpose of testing and comparing the above-described design with the conventional two resistive elements hot water heaters. The especially designed hot water tanks have five resistive elements to incorporate both the standard two elements heater with the herein described three elements peak power reducing water heater into one and only tank. The two elements design has two resistive heating elements each having a power rating of about 3800 Watts, and never operating simultaneously such that the maximum peak power demand on this water heater design never exceeds 3.8 kW.

The testing was done over 4 months, in 75 households all selected to best represent typical families. The hot water consumption and the electrical demand for two tank modes: the traditional two elements design and the proposed three elements design were measured.

For the purpose of the experiment, a timer was used to switch between operation modes, that is, from the operation of one design to the operation of the other design. The switching between the two operation modes was done late in the evening on a daily basis to allow a testing of both concepts for a same customer. The switching apparatus used is illustrated in FIG. 6. In addition, the thermostats in both operation modes were all set to 60 degrees Celsius or 140 degrees Fahrenheit.

The study was conducted during the winter time for approximately four months switching day to day around ten o'clock in the evening, from one type of element configuration to the other.

A survey done by Multi-Réso group shows how hot water was used amongst the families involved:

-   -   1) 3 persons in average per family;     -   2) 67% full time workers; 11% part time; 14% retired et 8% at         home;     -   3) Average use of Dishwasher: 3.7 per week;     -   4) Clothes Washing Machine: 6.5 per week. 66% cold water only;         32% warm and 5% hot water only;     -   5) On average, 17 showers per week and 4 baths per week; 61% of         customer having a low flow shower head.

Each site had two data logger Smart Reader™ that monitored the following parameters: Electrical power to water heater and hot water flow rate. A Watt-node™ accumulates the power and a given number of pulses are generated per kWh, Pulses are also output by the hot water flowmeter (1 pulse per liter). Data of pulses were stored every five minutes. A 30 amps CT was used to measure the input power at the water heater while a 5 amps CT only for the lower 800 Watts element. Indeed, the current can be as low as 3.3 amps when the water heater operates on its 800 Watts element (800 Watts divided by 240 Vac) and a low range CT must then be used to achieve accuracy.

The graph of FIG. 7 shows the bin destitution of daily hot water consumption for the three elements operation mode wherein roughly 5,000 data points were taken and averaged over all the customers involved in the testing. As stated hereinabove, seventy-five (75) residents participated in the study during roughly 4 months. Since the two tank modes were alternated on daily basis, each tank mode was tested for about 5,000 days, hence the 5,000 data points (one data point per day). FIG. 8 shows the cumulated daily hot water usage for each of the customers involved.

The graphs in FIG. 9 and FIG. 10 illustrate that a wide range of hot water usage profile exists for either one of the three elements operation mode and the two elements operation mode. The horizontal line in the two graphs shows the general trend for the average hot water usage in contrast with all the data points. Each data point indicates the daily use of hot water for one household. Overall, near 5,000 data points are plotted for each operation mode. An average of about 194 liters per day of hot water consumption is reached per household, in any operation mode.

These values are seen to fit well with other studies made relating to hot water consumption rates. For example, a survey of study done in 1998 by, Abrams, D. W. “Field Test Results From Residential Electric Resistance Water-Heating Systems,” ASHRAE Trans., 1998, Vol. 104, Part 1B, No. SF-98-31-1, 1843-1851, estimated average daily hot water consumption on 14 sites to 218.8 liters per day per household. A previous study by Perlman et Mills in 1985 (refer to Perlman, M. wt B. E. Mills (1984), “Development of Hot Water syue Patterns”, ASHRAE Transactions 91(2): 657-679), a study done in Canada estimated the average daily hot water to be 236 liters.

By using data from previous studies of Hydro-Québec and the survey done by Multi Réso-Senergis in January 2007 to estimate the habits of hot water usage per household, weekly hot water consumption was found to be of 1491 liters, or 213 liters daily, as shown in the following table from the Multi-Réso group.

Approximate calculation of daily use of hot water based on typical usage and habits of customer based on the survey of Multi Réso among the households of the pilot project Liters of kWh per Number of Hot water Hot water usage use liters at 60° C. per from 8° C. to per week per week Usage usage 60° C.* (B) (A × B) Bathing 57.5 liters 3.4 kWh 4 230 Shower 37.1 liters 2.2 kWh 17 631 Dishwashers 28.3 liters 1.7 kWh 3.7 105 Clothes 32.3 liters 1.9 kWh 6.5 210 Washing Machine Other uses   15 liters 0.9 kWh 7 315 per day per TOTAL per week: 1491 liters per day 213 liters *Does not take into account the standby losses **: Le Chauffage électrique résidentiel de l'eau: Canadian Electrical Association (CEA), 1990

Approximate calculation of energy consumption to heat water to 60° C. based on Hydro-Quebec's report LTE-RT-2006-0114 Average 1 2 3 4 ou+ household Daily hot water consumption at 60° C. in liter per day Bathing 20 33 59 79 53 Shower 8 liters/min 25 43 60 74 56 Dishwashers 8 11 15 21 16 Other uses 15 30 45 60 42 Clothes Washing 13 20 32 45 30 Machine Total per day 81 138 213 279 197 Daily hot water energy consumption (kWh)* Bathing 1.2 2.0 3.6 4.7 3.2 Shower 8 liters/min 1.5 2.6 3.6 4.5 3.4 Dishwashers 0.5 0.7 0.9 1.3 1.0 Other uses 0.9 1.8 2.7 3.6 2.5 Clothes Washing 0.8 1.2 1.9 2.7 1.8 Machine Total per day 4.9 8.3 12.7 16.8 11.8 *Energy requires to heat water from 8° C. to 60° C. without standby losses

As seen in the previous table, the 213 liters per day is quite close to the value of 194 liters per day measured in the field by the actual study. In an other study by De Oreo et Mayer (taken from De Oreo, W. B. et P. W. Mayer (2000), “The End Uses of Hot Water in Single Family Homes from Flow Trace Analysis”, Aquacraft Inc. Report, undated.) (10 houses from Seattle) in year 2000, 235 liters per day were measured. The Canadian study from Perlman and Mills in 1985 showed a daily hot water consumption of 236 liters per day (taken from Perlman, M. et B. E. Mills (1984), “Development of Hot Water use Patterns”, ASHRAE Transactions 91 (2): 657-679).

Comparison of the study from DeOreo et Mayer in year 2000 with the actual test pilot DeOreo et Hydro-Québec pilot Hydro-Québec Mayer project (year DeOreo pilot project (year 2006- and (year 2006- 2000) 2007) % of Mayer (year 2007) liters liters daily 2000) Usage per day per day usage % of daily usage Bathing 33 liters 41 liters 15% 18% Shower 90 liters 62 liters 42% 26% Dishwashers 15 liters  9 liters  7%  4% Clothes Washing 30 liters 38 liters 14% 16% Machine Other uses 45 liters 85 liters 21% 36% Total per day 213 liters  235 liters  (from Table 1)

Comparison of previous studies shows the average hot water consumption of this actual study to fit well within the range of previous studies. Since the Pearlman's study was done in 1984, one should now expect a reduction of the hot water consumption due to energy efficiency having been improved over the last decades. Furthermore, the use of low flow shower heads and faucets as well as the promotion of cold water for clothes washing also contribute to the reduction of hot water consumption when compared to earlier studies.

The graphs shown in FIG. 11 to FIG. 13 b illustrate that the diversified power demand for the herein proposed three element hot water tank exhibits a flatter behavior compared to the diversified power demand for a conventional two element hot water tank. A flatter diversified power demand is advantageous since the generation capacity demand and the electrical load are better managed.

As can be seen in the graph of FIG. 11, the three elements water heater exhibits a reduction in its diversified peak power demand of about 200 Watts, down to a value lower than 800 Watts. This reduction translates to about a 20% reduction in peak demand when compared to conventional hot water tanks having two resistive heating elements. This information was provided through the field study mentioned hereinabove which was conducted in 2007 by Hydro-Québec.

The mean, the maximum and the minimum values of the diversified power demand for each average daily hot water usage of a specific customer household, in both the morning and the afternoon peak power demand periods (7 h-11 h and 17 h-21 h), was calculated for the two elements operation mode (graphs of FIGS. 12 a and 13 a) and the three elements operation mode (graphs FIGS. 12 b and 13 b).

As the mean daily hot water usage increase, the demand rises. A curve fit is provided for the maximum, the mean and the minimum values. The following table summarizes the mean and maximum values for each peak power demand period, along with the power reductions seen for the three elements water heater design.

Two (2) Three (3) Period elements elements Reduction Mean Power (kW)  7 h-11 h 0.72 0.56 22% 17 h-21 h 0.83 0.70 16% Maximum Power (kW)  7 h-11 h 0.85 0.64 25% 17 h-21 h 0.94 0.78 17%

The proposed three element water heater therefore addresses the issues related to the prior art and offers efficient, simple and less costly solution to peak demand management without the drawbacks of unwanted peaks occurring when turning water heaters back on after long periods of inactivation.

It is within the ambit of the present invention to cover any obvious modifications of the preferred embodiment described herein, provided such modifications fall within the scope of the appended claims. 

We claim:
 1. A peak-power reducing domestic hot water heater comprising: a closed tank having a predetermined water holding capacity, a hot water outlet in a top end wall of said tank, a cold water inlet in a side wall of said tank adjacent a bottom wall thereof, three spaced apart resistive heating elements projecting substantially horizontally in said tank, each of said resistive heating elements having a temperature sensing element to enable a control of their respective activation; a bottom one of said resistive heating elements extending in said tank spaced slightly above said bottom wall and having a lowest power rating, said bottom resistive heating element being turned on when water is flowing into said cold water inlet and drawn out of said hot water outlet during one of the daily peak-power demand time periods; a middle one of said resistive heating elements extending in said tank at a level which is calculated to be equal or above an average maximum daily water consumption volume drawn out during the peak power demand time periods; and a top one of said resistive heating elements extending between said middle element and said top end wall of said tank and having a highest power rating; and said middle and said bottom resistive heating elements being activated simultaneously only when said top resistive heating element is inactive.
 2. The peak-power reducing domestic hot water heater as claimed in claim 1, wherein said bottom resistive heating element has a low watt density rating in the range of from about 15 to 30 W/in².
 3. The peak-power reducing domestic hot water heater as claimed in claim 1, wherein said average maximum daily water consumption volume drawn out during the peak power demand time periods is in the range of about 90 to 130 liters.
 4. The peak-power reducing domestic hot water heater as claimed in claim 1, wherein said average maximum daily water consumption volume drawn out during the peak power demand time periods is in the range of about 130 to 150 liters.
 5. The peak-power reducing domestic hot water heater as claimed in claim 1, wherein said tank is a cylindrical tank having a diameter of about 20 inches or less in order to produce an efficient water temperature stratification between hot and cold water in said tank and thereby reduce water temperature diffusion and ensuring a constant hot water supply in a top portion of said tank.
 6. The peak-power reducing domestic hot water heater as claimed in claim 5, wherein said bottom heating element has a life rating which is higher than said middle and top heating elements.
 7. The peak-power reducing domestic hot water heater as claimed in claim 6, wherein said bottom resistive heating element has a metal sheath made from one of copper, incoloy or stainless steel, and wherein a magnesium oxide surrounds a central coil thereof.
 8. The peak-power reducing domestic hot water heater as claimed in claim 5, wherein said middle resistive heating element produces a heat barrier in said water within said tank at the level of said middle resistive heating element in the range of about 140° F. to reduce the propagation of harmful bacteria to said hot water supply in said top portion of said tank.
 9. The peak-power reducing domestic hot water heater as claimed in claim 8, wherein said harmful bacteria is legionnela bacteria.
 10. The peak-power reducing domestic hot water heater as claimed in claim 1, wherein said tank has a water capacity of about 270 liters, said bottom heating element extends at a water level of about 20 liters from said bottom wall, said middle heating element extending at a water level of about 130 liters from said bottom wall and said top heating element extends at a water level of about 220 liters from said bottom wall.
 11. The peak-power reducing domestic hot water heater as claimed in claim 1, wherein said tank has a water capacity of about 270 liters, said bottom heating element extends at a water level of about 20 liters from said bottom wall, said middle heating element extending at a water level between about 130 liters to about 150 liters from said bottom wall and said top heating element extends at a water level between about 220 liters to about 240 liters from said bottom wall.
 12. The peak-power reducing domestic hot water heater as claimed in claim 11, wherein said top heating element is spaced from said top wall of said tank a distance equivalent to a water volume of about 46 liters.
 13. The peak-power reducing domestic hot water heater as claimed in claim 1, wherein said bottom resistive heating element is a bolt-on resistive heating element.
 14. The peak-power reducing domestic hot water heater as claimed in claim 1, wherein said water holding capacity of said tank is one of about 180 liters and about 270 liters.
 15. A method of reducing the kilowatt demand of a domestic hot water heater during peak hour periods without reducing the amount of hot water requested by a user from a hot water tank having a predetermined water holding capacity, said method comprising the steps of: providing a hot water tank with three spaced-apart electrical resistive heating elements extending therein in a spaced-apart manner and each having a thermostat to enable a control of their respective activation; positioning a middle one of said resistive heating elements at a level exceeding an average maximum daily water consumption volume drawn during one of the peak hour periods, and activating said bottom one of said resistive heating elements when water is flowing into said cold water inlet and drawn out of said hot water outlet during the peak hour periods, and simultaneously activating said middle and said bottom one of said resistive heating elements only when said top resistive heating element is inactive.
 16. The method as claimed in claim 15, wherein said step of providing comprises providing said bottom one of said resistive heating elements with a low watt density rating in the range of from about 15 to 30 W/in².
 17. The method as claimed in claim 15, wherein said step of positioning comprises positioning said middle one of said resistive heating elements at a level exceeding an average maximum daily water consumption volume drawn out of the tank during the peak hour periods at a level between about 90 liters to about 130 liters depending on said tank size of predetermined volume.
 18. The method as claimed in claim 17, wherein said step of positioning comprises positioning said middle one of said resistive heating elements at a level exceeding an average maximum daily water consumption volume drawn out of the tank during the peak hour periods at a level between about 130 liters to about 150 liters depending on said tank size of predetermined volume.
 19. The method as claimed in claim 15, wherein there is further provided the step of operating said middle resistive heating element to maintain a water temperature at its said level at a temperature of about 140° F. to form a temperature barrier to reduce the propagation of harmful bacteria from the bottom of the tank towards a top portion of the tank where hot water is drawn.
 20. The method as claimed in claim 15, wherein said water holding capacity of said tank is one of about 180 liters and about 270 liters. 